The present invention relates to signal processing in a pulse oximeter used in a patient monitoring or for diagnosing a respiratory system or a circulatory system.
Various methods have been proposed to separate a signal component and a noise component from two signals substantially simultaneously extracted from a single medium. The methods are usually conducted by performing processing in a frequency region and a time region.
In medical fields, it is known a pulse photometer including: an apparatus called a photoplethysmograph for measuring a pulse waveform and a pulse rate; and an apparatus for measuring the concentration of a light absorbing material in blood such as an apparatus for measuring an oxygen saturation in blood (SpO2), an apparatus for measuring the concentration of abnormal hemoglobins such as carboxyhemoglobin and methemoglobin, and an apparatus for measuring the concentration of an injected dye. Particularly, the apparatus for measuring the SpO2 is called a pulse oximeter.
The principle of the pulse photometer is to determine the concentration of an object material from a pulse wave signal obtained by causing a living tissue to transmit or reflect light beams, which have a plurality of wavelengths respectively correspond to different absorbances of the object material, and by continuously measuring an intensity of transmitted or reflected light. In a case where noise are superimposed on pulse wave data, calculation of a correct concentration or pulse rate cannot be achieved. Consequently, there is an anxious that an erroneous treatment may be performed.
There is proposed a method performed in the pulse photometer, in which pulse wave signals are divided into a plurality of frequency bands and a correlation between the signals is examined in each of the divided frequency bands to reduce noises. However, this method has a problem in that analysis is time-consuming.
Japanese Patent No. 3270917 discloses a method comprising: irradiating light beams respectively having different wavelengths onto a living tissue; drawing a graph in which the longitudinal axis and the transverse axis thereof respectively represent the magnitudes of two signals obtained from transmitted light; and obtaining a regression line to thereby obtain an oxygen saturation or the concentration of a light absorbing material in arterial blood according to the gradient of the regression line.
However, a large amount of calculation is needed to obtain a regression line and the gradient thereof by using a great amount of sampling-data on each of the pulse wave signals.
Japanese Patent Publication No. 2003-135434A discloses a method of using frequency analysis, in which the fundamental frequency of a pulse wave signal is obtained instead of conventional extraction of the pulse wave signal itself, and the pulse wave signal is filtered by using a filter employing a higher harmonic wave frequency to enhance accuracy. Japanese Patent Publication Nos. 2005-95581A and 2005-245574A disclose a method for separating noise from a pulse wave signal.
However, in a case where noise due to a body movement of a patient, whose amplitude is about ten times that of the pulse wave, is superimposed on a pulse wave signal, it is difficult according to any of these methods to compute a pulse rate and an oxygen saturation in arterial blood. Thus, a further improvement in signal processing is desired.